This command will close all unknown connections to your server. You can list those connections with "STATS U".
You normally won't need to close these connections, but if your server is accumulating a large number of unknown connections, then you may want to clear them all off so that other users can join.
Note: some clients implement an own CLOSE command, which has nothing to do with the server command. If that is the case you need to tell the client to send the command to the server, often done with something like "/raw CLOSE".